(1) Field of the Invention
The present invention relates to the production of a base sheet to be surface-treated, that is, a steel sheet as a base steel sheet to be plated for a surface-treated sheet such as tinplate and tin free steel in which a steel sheet is thinly plated with Sn or Cr, and is to effectively avoid the occurrence of the stretcher strain in the treatment, particularly drawing, made on the surface-treated steel sheet.
For instance, according to the JISG3303, the tempering degree is classified into several ranges from T-1(HR30T:49.+-.3) to T-6(HR30T:70.+-.3) depending upon intended Rockwell T hardnesses (HR30T). Such classification is made with respect to the box annealing, and in particular the classification from the T-4-CA to T-6-CA (HR30T:61.+-.3 to 70.+-.3) is specified with respect to the continuous annealing. The present invention is particularly suitable for the tinplate having the tempering degree of T2 or higher among the above-mentioned classification ranges and tin-free steel similar thereto.
(2) Description of the Prior Art
As the base steel sheet of T-1 to T-4 grades to be plated as tinplate, there has been heretofore mainly used a low carbon aluminum-killed steel having 0.01 to 0.10% by weight (hereinafter also referred to briefly as "%" with respect to the other components of the steel), while as the base sheet of T-5 and T-6 grades, use has been principally made of a low carbon aluminum-killed steel in which P or N is added to increase the hardness.
The relation between the annealing method performed on the base steel sheet to be surface-treated and the properties of the tinplate is as follows: Box annealing:
Since cooling is gradually performed down near room temperature in a few or several days after recrystallization (550.degree.-700.degree. C.), most of carbon in the steel precipitates as carbide. On the other hand, nitrogen in the steel precipitates as aluminum nitride during heating.
That is, since C and N in the steel are not present in solid-solution state, even when the temper rolling and plated tin-alloying treatment (a so-called reflow treatment in which the steel is maintained at 230.degree.-250.degree. C. for a few seconds) after tin-plating are carried out, strain aging does not occur to cause no yield point elongation.